* Make CoreDNS a server type plugin for Caddy
Remove code we don't need and port all middleware over. Fix all tests
and rework the documentation.
Also make `go generate` build a caddy binary which we then copy into
our directory. This means `go build`-builds remain working as-is.
And new etc instances in each etcd test for better isolation.
Fix more tests and rework test.Server with the newer support Caddy offers.
Fix Makefile to support new mode of operation.
Make middleware that sets up a (http) handler survive a graceful
restart. We calls the middleware's Shutdown function(s). If restart
fails the Start function is called again.
* middleware/health: OK
* middleware/pprof: OK
* middleware/metrics: OK
All restart OK.
